There were invisible guards what
I found an invisible guard in the courtyard of the Castel Sant'Angelo.
I was jogging and suddenly Ezio stumbled as if he'd bumped into somebody. After doing this several more times, I determined that there was an invisible NPC standing there.
I approached the anomaly and pressed X with the hidden blade equipped, so Ezio performed a stealth assassination on an invisible being. Complete with a disembodied blood spurt.
I heard the thud of a body hitting the ground, and looted him. A throwing knife and some medicine; he was a guard.
Given the option to pick up his weapon, Ezio held it in two hands.
Seeing a visible guard from a distance and targeting him, I held X and released to throw the invisible weapon at the guard.
A split second later, he fell backward, dead.
I had mime-killed a guard.
I felt so badass.